1. Field of the Invention
The present invention relates generally to a digital pulse-width-modulated pulse generator and more specifically to the pulse generator for generating a PWMed pulse signal synchronizing with a sine wave by PWMing a sine wave signal in dependence upon a triangular wave signal in digital fashion. Further, the digital PWMed pulse generator according to the present invention is advantaneously incorporated with a decoupled-vector control induction motor driving system by way of typical example.
2. Description of the Prior Art
Conventionally, an analog PWMed pulse generator has generally been used for generating a PWMed pulse synchronizing with a sine wave. In the prior-art pulse generator, however, since the amplitude of a sine wave signal is compared with that of a triangular wave signal in analog fashion by a comparator, whenever ambient temperature varies, the amplitudes of both the signals change and further an offset voltage of the comparator inevitably fluctuates. Therefore, it is impossible to generate an accurate PWMed signal. In particular, where the voltage and the frequency of the PWMed pulse is small, it is very difficult to generate a PWMed signal accurately. Of course, it may be possible to eliminate the harmful influence of variation in ambient temperature upon the generator in some way. However, the generator may be increased in size, cost, the number of elements, etc.
On the other hand, the method of driving an induction motor has been highly developed owing to a remarkable progress in power electronic device technology. Especially, the vector control method has been proposed for driving an induction motor at variable speeds under quick response characteristics equivalent to a DC machine. In this vector control method, the primary current of an induction motor is divided into a primary exciting current to generate the secondary magnetic flux and a primary driving current to generate the secondary driving current, and further the vectors of the secondary magnetic flux and the secondary driving current are so controlled independently as to meet at right angles to each other. Further, in this vector control method, the magnitude of the secondary magnetic flux is controlled at a constant level and the secondary driving current is increased or decreased independently as in a DC motor. In the above-mentioned vector control method of driving an induction motor, however, since there exists a mutual interference between the secondary magnetic flux and the secondary driving current, the magnitude of the secondary magnetic flux is not maintained constant in practice. To overcome this problem, the so-called decoupled-vector control method is adopted, in which the mutual interference or the vector cross-term between the secondary magnetic flux and the secondary driving current is cancelled out. Theoretically there are three necessary and sufficient conditions in order to decouple two vectors of the secondary magnetic flux and the secondary driving current. These conditions are usually satisfied by adding a decoupling calculation unit to the ordinary vector control system.
In the above-mentioned decoupled-vector control method or system for an induction motor, conventionally, the induction motor is driven by an inverter activated by a PWMed pulse (for each phase). In the prior-art decoupled-vector control method or system, however, since the PWMed pulse is generated in analog fashion by comparing a sine wave signal generated by an analog sine wave generator with a triangular wave signal generated by an analog triangular wave generator, it is difficult to generate an accurate PWMed pulse. In particular, when a phase difference command signal is applied to the control system, it is impossible to control the phase difference at high response speed on the basis of relatively simple calculations.